chore: update README to add an example (#30)

Signed-off-by: Xiaoxuan Wang <wangxiaoxuan119@gmail.com>

README.md

@@ -18,8 +18,7 @@ steps:
 
 ## Advanced Usage
 
-Install a specific version of the `oras` CLI by specifying the input `version` without the prefix `v`.
-Supported versions can be found at [`oras` releases](https://github.com/oras-project/oras/releases).
+Install a specific version of the `oras` CLI by specifying the input `version` without the prefix `v`. Stable versions (not pre-release versions) are supported and can be found at [`oras` releases](https://github.com/oras-project/oras/releases).
 
 For example, install `oras` version `v1.0.0`.
 
@@ -31,6 +30,19 @@ steps:
   - run: oras version
 ```
 
+You can also download `oras` CLI from any preferred trusted source, by specifying link of the source via `url` and the SHA256 checksum via `checksum`. This can be used to download a pre-release version or download from a proxy other than `github.com`. 
+
+For example, install `oras` version `v1.2.0-beta.1`.
+
+```yaml
+steps:
+  - uses: oras-project/setup-oras@v1
+    with:
+      url: https://github.com/oras-project/oras/releases/download/v1.2.0-beta.1/oras_1.2.0-beta.1_linux_amd64.tar.gz
+      checksum: 37d86f848f7c7c471035cf8218f06372fd72a9fbdca1e10f509738e222b3b2be
+  - run: oras version
+```
+
 ## Docs
 
 Documentation for the ORAS CLI is located on

action.yml

@@ -22,7 +22,7 @@ inputs:
     required: false
     default: 1.2.0
   url:
-    description: URL of the customized ORAS CLI to install
+    description: URL of the customized ORAS CLI to install. Required if 'checksum' is present.
     required: false
   checksum:
     description: SHA256 of the customized ORAS CLI. Required if 'url' is present.